Jellyfish

Jellyfish are ancient animals that have been adrift, gliding along in the world seas for more
than 500 million years. Carried by waves and sea currents, they are present in all of the
world’s oceans, although freshwater species do also exist. These organisms are free-
swimming forms of the Medusozoan clade (phylum Cnidaria) with an umbrella-shaped,
radially symmetric body plan, representing the adult, sexually mature forms that emerge
following the asexual polyp phase, which in turn is sessile, ie. polyps live attached to a
substrate. With content of their bodies being up to 98 % water, jellyfish medusae move
about aided by the motion of sea water. Virtually all jellyfish feed on plankton and tiny
marine animals, which they capture using their tentacles armed with sting cells. These
cnidocytes (cells) contain characteristic organelles – ‘cnidae’, which are in fact capsules
consisting of a trigger, a spring and a tiny dagger-like sting on top. Upon mechanical or
chemical stimulation, a cnida is activated, the prick is thrust forward and a neurotoxin
injected into the target organism, thus paralysing it. Nonetheless, jellyfish too have their
natural enemies. Namely, certain fish and sea turtle species relish having them on the menu.

„Upside-down“ jellyfish(Cassiopea andromeda,Forskål 1775.)

„Upside-down“ jellyfish is an interesting cnidarian that lives in shallow lagoons surrounded by
mangroves, on beds of sand or silt. The body of the jellyfish, of yellowish-brown coloration,
pulsates in order to generate a flux of water bringing oxygen and food to it. „Upside-down“
jellyfish is a carnivore whose diet consists of smaller organisms which it, prior to being
devoured, paralyses using stinging cells and mucous secretion. It lives in “endosymbiosis” with Zooxanthellae, single-cell algae present in the tissue of the jellyfish. Akin to other
cnidarians, the life cycle of „upside-down“ jellyfish is marked by sexual and asexual phases
of reproduction. While in its sedentary (polyp) form, it reproduces by budding (asexually). In
the ensuing maturation phase, the mature, reproductively competent „upside-down“ jelly
develops eggs and keeps them stored until the male gametes are discharged in nearby
water. The female then fertilises her eggs by drawing in the male gametes with her tentacles.

Moon jellyfish(Aurelia aurita,Linnaeus 1758.)

Moon jellies often aggregate in large groups and are commonly found in all the oceans. They
are easily recognised by the presence of four moon-like rings on top of their bell-shaped
bodies. These rings are actually reproductive organs of the jellyfish and their sex can be
determined accordingly: the male rings are white, while the female are pink. They have short
tentacles and four small „arms“ near their mouths, which are used to capture prey on which
they feed. The moon jellyfish stinging tentacles are venomous to smaller marine organisms,
but are not dangerous to humans since the venom cannot penetrate the skin. Moon jellyfish
have an interesting life history, because like other cnidarians, they too go through distinct
sexual and asexual phases. During mating, the female draws in the male gametes through
her mouth, such that the subsequent fertilisation takes place internally. Once the free-
swimming larvae hatch, they sink and attach themselves to the substrate. Next, from the
larvae, small sedentary polyps develop and having undergone asexual reproduction, the
polyps release free-swimming immature jellyfish (ephirae). Ephirae reach sexual maturity
after three months and the cycle is then repeated again.